Communication systems typically require mechanisms to provide synchronization at the receiver. Such mechanisms are in particular required for orthogonal frequency division multiplexing (OFDM), a communication scheme for transmitting data over N parallel sub-carriers, which is used within the context of Digital Video Broadcasting (DVB) systems, certain wireless local area networks, digital radio systems, power line communications as well as other systems.
An OFDM symbol of length L=N+G consists of N (complex) time-domain samples emerging from an OFDM modulator, as well as a cyclic extension of length G. The cyclic extension is a portion of the N samples taken from the end and/or beginning and positioned preceding and/or following the N samples. The cyclic extension may be utilized as an aid in distinguishing one OFDM symbol from another, as well as function as a guard interval for mitigating inter-symbol interference (ISI) and other undesirable propagation effects.
For an OFDM symbol to render any useful information, an OFDM receiver must be able to distinguish the OFDM symbols. A correlation technique is normally used to determine a starting point of an OFDM symbol. However, in certain communication systems, including DVB-T systems, the precise length of an OFDM symbol is not known to a receiver. Common methods of determining the length of the symbol and estimating its position have been to guess a certain symbol length, perform correlations of the received signal, and to see how the correlation changes over time. If the correlation increases beyond a threshold level, the OFDM symbol is deemed to have been found. While likely to ultimately succeed, this brute force method is time consuming and susceptible to various error sources. Moreover, these random guesses may also not provide a sufficient result in a reasonable amount of time.